Generally, a power supply unit (a power supply apparatus) that performs AC/DC conversion of a commercial alternating-current power supply to generate a direct-current internal power supply is incorporated in a programmable controller (PLC) that controls an industrial machine. A function of performing power monitoring is sometimes incorporated in such a power supply unit so as to be used for performing monitoring related to an energy-saving operation and analysis for finding abnormalities. For example, Patent Literature 1 discloses a technique related to a power supply unit that measures an input voltage, an input current, an output voltage, an output current, and a temperature within the power supply unit and that outputs these values.
However, when it is attempted to directly measure input power, a large and expensive part such as a voltage transformer (VT) or a current transformer (CT) needs to be incorporated, which leads to disadvantages of product distribution.
To handle this problem, Patent Literature 2 discloses a technique of measuring an output voltage and an output current and then calculating input power by using the measured output voltage and output current and a conversion efficiency of AC/DC conversion obtained by being measured in advance (hereinafter, sometimes simply “efficiency”). According to this technique, because the input power is supposed to be indirectly determined by the output power and the efficiency, the part mentioned above is not required.